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MODULE D2 – CELLULAR NETWORKSmobnet.epfl.ch
Some of the slides are adapted from Stallings, Wireless Communications & Networks, Second Edition, Chapter 10
© 2005 Pearson Education, Inc. All rights reserved. 0-13-191835-4
Some of the content is inspired by Rappaport, Wireless Communications, Second Edition, Chapters 3 and 9, 2002
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Public Switched Telephone Network - PSTN (reminder)
Localswitch
Localswitch
Transitswitch
Outgoing call
Incomingcall
Transitswitch
Transitswitch
Long distance network
- Transfer mode: circuit switching- All the network (except part of the access network) is digital- Each voice channel is usually 64kb/s
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PSTN Trunk Dimensioning (reminder)
switchN channels
A
Assumptions• Loss system: if the N channels are busy, any additional call is dropped • Independent sources
1 PrBlocking AN
[Erlangs]A E X
where X = call duration [sec/call]Y = call arrival [calls/sec] ~ Poisson()
0
Pr Pr("call dropped because line busy") Erlang-B( , )!
!
N
Blocking iN
i
AA NANi
Each channel N carries a traffic
switchoffered load
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Basic Call (reminder)Calling terminal Network Called terminal
Off-hook
Dial tone
Dialing
Ring indication Alert signal
Off hookRemove ring indication
Bi-directional channel
On hook
Billing
On hook signal
Resource allocation
Translation + routing
Conversation
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Architecture of Cellular Networks
ExternalNetwork
Cellular Network
MobileStation Base
StationMobile
SwitchingCenter
Server(e.g., Home Location
Register)
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7
Registration
Tune on the strongest signal
Nr: 079/4154678
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Service Request
079/4154678079/8132627 079/4154678
079/8132627
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Paging broadcast
079/8132627?079/8132627?
079/8132627?
079/8132627?
Note: paging makes sense only over a small area
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Response
079/8132627
079/8132627
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Channel Assignment
Channel47
Channel47 Channel
68
Channel68
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Conversation
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Handover (or Handoff)
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Message Sequence Chart
CallerBase
StationSwitch Base
Station Callee
Periodic registration Periodic registration
Service request Service request
Ring indicationRing indication
Page requestPage requestPaging broadcast Paging broadcast
Paging responsePaging response
Assign Ch. 47Tune to Ch.47
Assign Ch. 68 Tune to Ch. 68
Alert tone
User responseUser responseStop ring indicationStop ring indication
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Peculiarities of Cellular Networks Mobility
User location => periodic registration and/or paging Moving from a cell to another => handoff (US) or handover
(UK) procedures Moving from one network to another => roaming
Ether Multiple users per cell => access technology (e.g., SDMA,
FDMA, TDMA, CDMA) Channel impairments => coding, error detection,
retransmission, forward error correction Bandwidth => channel reuse, signal compression, efficient
modulation and coding Privacy and security => encryption
Energy Limited autonomy => power control, discontinuous
transmission
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Offered Services Telephony services (i.e., voice mail, call transfer,…)
Short Message Services (SMS)
Packet switched data (e.g., GPRS, EDGE, HSDPA, LTE), notably for Web access
Location-based services
Application store (AppStore of Apple, Application Market of Android,...)
Entertainment (music, video,…); Mobile TV
Mobile extension of online social networks (Facebook Mobile,…)
Friend location (Foursquare, Google Latitude, LocaliserMesAmis,…)
Peer-to-peer wireless services (e.g., over Bluetooth and WiFi in ad hoc mode); NIC (Nokia); FlashLinQ (QualComm)
…
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Relevant Service FeaturesUser Perspective
Terminal characteristics Weight, size, robustness Price Battery life User interface
Network characteristics Coverage area (of home network + roaming agreements) Call blocking/dropping Transmission quality (error rate, signal to distortion ratio, delay)
Service characteristics Price Range of services Confidentiality, Authentication and Privacy
Relevant Service FeaturesOperator Perspective
Efficiency Spectrum efficiency Frequency reuse Cell radius
Cost Infrastructure cost Deployment time and adaptability Roaming agreements
Security Resistance to fraud Non-repudiability
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MHzcellsonsconversatiE
For telephony:
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Air Interface
Messages
Logicalchannels
Radio link
Messages
Logicalchannels
Radio link
Packets
Messages
Bits
Users’ data
Packet structure, error detection/retransmissionTopology: one to one
one to many (e.g., synch signals)many to one (e.g., service request)
Multiple access (e.g., CDMA, TDMA, FDMA)Duplex (e.g., Frequency Division Duplex - FDD)Modulation, source coding, channel coding,interleaving, diversity, channel equalization
Terminal Base Station
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Cellular Networks
Covered area tesselated in cells• One antenna per cell• Cells are controlled by Mobile
Switching Centers
A mobile communicates with one (or sometimes two) antennas
Cells are modeled as hexagons Cells interfere with each other
To increase the capacity of the network, increase the number of cells
Generations of Cellular Networks…
1G: analog systems not in use anymore 2G: GSM (introduced in 1992): FDMA/TDMA (900 and
1800MHz) 2.5G: with GPRS: packet switching, extended to E-GPRS (nicknamed
EDGE)
3G: UMTS (introduced in 2002): CDMA (2100 MHz) 3.5G: with HSPDA (up to 14.4Mb/s); with HSPA+ (up to 84Mb/s)
4G: LTE (being introduced in 2013): OFDMA (800 and 2600MHz, then technology neutrality); up to 100Mb/s
GPRS: General Packet Radio ServiceHSPDA: High Speed Downlink Packet AccessLTE: «Long Term Evolution»For more information: see the 3GPP standards
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21.5 3RArea of the hexagon:
Distance between adjacent cells: 3d R
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Frequency Reuse
F3F4
F5
F2F7
F6F1
F3F4
F5
F2F7
F6F1
F3F4
F5
F2F7
F6F1
Channel assignment strategies• Fixed: each cell is allocated a predetermined set of channels• Dynamic: each time a call request is made, the serving base station
requests a channel from the MSC
Cells with the same nameuse the same set of frequencies
Cells are organized into clustersIn this example, the cluster size N = 7
In order to tesselate, the geometry of hexagons is such that N can only havevalues which satisfy
N = i2 + ij + j2
with i = 0,1,2,… and j = 0,1,2,…
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N: cluster size
i=2, j=0i=2, j=1
i=3, j=2
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Example: system of 32 cells with cell radius of 1.6kmTotal frequency bandwidth supporting 336 traffic channelsReuse factor (or cluster size) = 7What geographic area is covered?Total number of supported channels?
Solution:Cell area = 6.65km2Covered area: 32*6.65=213km2Channels/cell = 336/7=48Total channel capacity: 32*48=1536 channels
Same question for a system of 128 cells with cell radius of 0.8km. As before: - total frequency bandwidth supporting336 traffic channels
- reuse factor (or cluster size) = 7
Solution:Cell area: 1.66km2Covered area: 128*1.66=213km2Total channel capacity: 128*48=6144
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Rate of calls per minute: 97/60Average holding time per call: 294/97 Offered traffic: 294/60= 4.9 Erlangs
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Interference & System Capacity
Sources of interference Co-channel interference (same frequency)
– A call in a neighboring cell – Other base stations operating in the same frequency band– Non-cellular system leaking energy into the frequency band
Adjacent channel interference (adjacent frequency)– Another mobile in the same cell
Consequences of interference On data channel:
– Crosstalk (voice) – Erroneous data (data transmission)
On control channel: – Missed/dropped calls
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Decibels (reminder)
10110 log 20
100dB
The decibel is a dimensionless unit used to express a power ratio
where P0 is the reference power level and P is the considered power level
Decibel (dB) • express the magnitude of a physical quantity relative to a reference level. • represent very large range of ratios• are easy to manipulate (e.g., consecutive amplifiers)
A ratio• can be expressed in decibels relative to 1 Watt (dBW) • is more frequently expressed in decibels relative to 1mW (dBm)
Example: If the transmission power P0 is 10W and the received power P is 0.1W, the loss is
100
10 log PBP
1010 log1
PPmW
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Co-channel Interference (1/4)
0 00 0
or (dBm) (dBm) 10 logr rd dP P P Pd d
Co-channel reuse ratio Q
where D = distance to the center of the nearest co-channel cellR = radius of a cellN = cluster size (or “reuse factor”)
Signal-to-interference ratio (SIR)
where S = desired signal powerIi = interference power caused by the ith interfering co-channel base stationi0 = number of co-channel interfering cells
Average received power Pr at a distance d from the transmitting antenna
where P0 = power received at a small distance d0 from the transmitting antennaα = path loss exponent
3DQ NR
0
1
i
ii
S SSIRI
I
F5
F5R
D
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Co-channel Interference (2/4)
0 0
3( ) NS D RI i i
If the transmit power of each base station is equal and α is the same throughout the coverage area, in a corner of a cell (most remote place from the base station in the cell) we have:
Considering only the first layer of interfering cells and assuming that they are equidistant from the desired base station (all at distance D):
0
1
i
ii
S RI D
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Co-channel Interference (3/4)
A
R
D-R
D-R
D
D+R
D+R
D
First tier of co-channel cells for a cluster size of N=7Note: the marked distances are approximations
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Co-channel Interference (4/4)
12( 1) 2 2( 1)
SI Q Q Q
Approximation of the SIR at point A
Using the co-channel ratio
Numerical example: If N=7, alpha = 4, then Q~4.6 and
2( ) 2 2( )S RI D R D D R
49.56 17.8 S dBI
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Capacity of Cellular Networks (1/2)FDMA/TDMA
0min
1
16i
ii
S R R SI D ID
1/
min
6 SQI
FDMA/TDMA capacity is bandwidth limited
Consider the downlink channel interference. Assume that the mobile is located at the edge of the cell. Consider only the interference from the first tier of co-channel cells (6 cells if N = 7).
We want the SIR to be greater than a given minimum SIRmin
Using the co-channel reuse ratio and because Q=D/R:3Q N
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Capacity of Cellular Networks (2/2)FDMA/TDMA
2 2/
/2min
63 3
t t
c c
B BmQ SB B
I
Techniques to improve capacity• Cell splitting• Sectoring
Radio capacity of cellular network
where Bt is the total allocated spectrum for the system Bc is the channel bandwidth
Using the co-channel reuse ratio
t
c
BmB N
radio channels/cell
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Capacity of Cellular NetworksCDMA
CDMA capacity is interference limited
Techniques to reduce interference Multi-sectorized antennas Discontinuous transmission mode (takes advantage of
intermittent nature of speech); duty factor between 3/8 and ½.
Power control: for a single cell, all uplink signals should be received approximately with the same power at the base station
Pilot signal: transmitted by the base station; used by each mobile to set its own power (for the uplink)
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CDMA Capacity: single cell case (1/2)
Let N = number of usersS = power of the signal received at the base station from a single user
Bit energy to noise ratio
where R = bitrateW = available bandwidthN0 = noise spectral density
Taking the thermal noise η into account
Thus, the number of users that can access the system is
1( 1) 1
SSNRN S N
0
/ /( 1)( / ) 1
bE S R W RN N S W N
0
/( 1) ( / )
bE W RN N S
0
/1 - /S/b
W RNE N
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CDMA Capacity: single cell case (2/2)
0
1 /1/s
b
W RNE N
To increase this number, 2 main techniques:- Leverage on the sporadicity of users’ activity (e.g., switch off a user while he does not talk)- Antenna sectorization
Let δ = duty cycle (or factor) of voice (typically between 3/8 and ½)Ns = number of users per sector
If the number of users is large and thermal noise is neglected:
0
/( 1) ( / )
b
s
E W RN N S
CDMA Capacity: multiple cells case (1/3)
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B0 controls the transmit power of its in-cell users, but not that of users in neighboring cells
Frequency reuse factor on the uplink
where N0 = total interference power received from N-1 in-cell usersUi = number of users in the ith adjacent cellNai = average interference power from a user located in the ith adjacent cell
Average received power from users in adjacent cell is computed as
where Nij = power received at the base station of interest from the jth user in the ith cell
0
0 i aii
NfN U N
/ai ij ij
N N U
B0
B6
B5
B4
B3
B2
B1
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CDMA Capacity: multiple cells case (2/3) Concentric circular geometry
d0
Consideredcell
R
2R+d02R-d0
3R
2d0
Adjacent cell
q1
M1 : number of wedge-shaped cells of the firstsurrounding layer of cells
A1 : area of the firstsurrounding layer
A1 = M1 A
To let all cells have thesame size A, we must have:M1 = 8q1 = 450
By recursion, for the ith layer:Ai = i8Aqi = p/4i
Firstsurroundinglayer
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CDMA Capacity: multiple cells case (3/3)
d0
R
2R+d0
2R-d0
3R
q
Innersublayer
Outersublayer
d
d’
0
0
0
22 2
0
22 2
For the inner sublayer, namely for (2 1) (2 ) (case depicted in the figure):
' sin 2 cos
For the outer sublayer, namely for (2 ) (2 1) :
' sin cos 2
Inter
i R d i R d
d d Ri d d
i R d d i R
d d d Ri d
0
0, , 0 0 0 0
ference power at B from the th subscriber of the th cell :
( , , ) ( '/ ) ( / )
In practice, the frequency reuse factor for CDMA is in the order of 0.3 to 0.7 (as a comparison, in the ca
i j
j i
P r d P d d d d
f
se of FDMA with cluster size = 7, = 1/7). f
Note: i is the layer number (i=1 if we consider only the first layer)
Interfering cells
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Conclusion
In this Module D2, we have addressed essentiallynetwork capacity
Cellular networks: many base stations Capacity can be increased notably by cell splitting
and cell sectoring Reminder: Division multiple access used in cellular
network generations (all with SDMA, of course): 2G: GSM: FDMA/TDMA 3G: UMTS: CDMA 4G: LTE: OFDMA (Orthogonal Frequency-Division Multiple
Access) for the downlink and SC-FDMA (Single-carrier Frequency Division Multiple Access) for the uplink
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References
Agrawal & Zeng: Chapter 5
T. Rappaport: Wireless Communications, 2nd edition, PrenticeHall, 2001
M. Schwartz: Mobile Wireless Communications, Cambridge University Press, 2005
W. Stallings: Wireless Communications and Networks, 2nd
edition, Prentice Hall, 2005, Chapter 10
Schiller, Chapter 4
